Check the vac. line from the Vac pump off the cylinder head to the intake to the brake booster - if this holds a vac. (should be able to suck one way or blow one way Only) then I would suspect the Brake Booster has a leaking diaphragm... would if this is bad check the inside for oil residue. Even if there isn't - it's always good to replace the booster with the vac. pump. Reason being - it will later become an external oil pump and fill the booster and cause it to fail again.